Mostly tender-hearted relatives (3)
I believe the answer is:
kin
'relatives' is the definition.
(kin can mean relatives or family)
'mostly tender-hearted' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'tender-hearted' becomes 'kind' (I've seen this before).
'kind' with its last letter taken off is 'KIN'.
(Other definitions for kin that I've seen before include "Family and relations" , "Relatives from Kinnegad" , "A person's relatives, goes with kith" , "Flesh and blood" , "One's family/relations" .)